1. Field of the Invention
The present invention relates to a pump apparatus which makes it possible to discharge a constant amount of a fluid all the time by controlling the flow rate of the fluid by rotation of a driving source.
2. Description of the Related Art
A constant rate discharge pump has been adopted in order to supply a constant amount of a chemical solution, a paint, a washing solution, or the like, for an apparatus of producing semiconductors or the like, a painting apparatus, and a medical apparatus.
A bellows type pump is often used for the constant rate discharge pump. In the bellows type pump, the suction pressure and the discharge pressure are obtained by expanding/contracting bellows surrounding a shaft member, driven by a motor or the like.
In this apparatus, the shaft member is displaced in the axial direction by the driving source such as the motor. The tip section of the shaft member is displaced in a pump chamber which is formed in a pump housing. The bellows is interposed between the tip section and the pump chamber, and the bellows is expanded/contracted when the tip section is displaced. The suction pressure is generated when the bellows is contracted in the pump chamber. Accordingly, liquid is sucked from the outside, and the pump chamber is filled with the liquid. On the other hand, the discharge pressure is generated by expanding the bellows in the pump chamber. Accordingly, the liquid is discharged from the pump chamber to the outside (see, for example, Japanese Laid-Open Patent Publication No. 10-47234).
In the case of the conventional constant rate discharge pump, when the flow rate of the fluid to be sucked and discharged is increased, it is necessary to set a large stroke of the shaft member and the tip section in the axial direction in response to the flow rate. In such a situation, the bellows needs to be large, which is expanded/contracted in conformity with the increase of the stroke amount. However the production cost becomes expensive, because the bellows is expensive.
When the flow rate of the fluid to be sucked and discharged is increased, the amounts of expansion and contraction of the bellows are increased. As a result, some pulsation may occur in the fluid when the fluid is discharged from the pump chamber to the outside.
Further, when a liquid is sucked into the pump chamber, the liquid remaining in the pump chamber after discharging the liquid from the pump chamber to the outside may be pooled on the outer circumferential surface of the bellows.